In the oilfield, when drilling for oil or gas, a platform is used to support a circular rotary table. Rotational energy is supplied to the rotary table through motors or the like, moving the rotary table in a circular fashion. The rotary table comprises a central kelly bushing which provides a central opening or bore through which a drill string passes. The kelly bushing typically provides four “pin holes” receptive of pins on a master bushing which when interlocked with the kelly bushing, drive a kelly held therein. The rotary table, kelly, master bushing and kelly bushing are art terms referring to the various parts of the drilling rig which actually impart the needed rotational force to the drill string to effect drilling. Such well drilling equipment is known in the art.
When adding or removing a joint of pipe from the drill string, wedges called “slips” are inserted into a bowl, called a slip bowl, in the central opening of the rotary table. The slips hold the drill pipe to prevent it from falling into the well bore. The placement of the slips may be manual, in which case the slips are provided with handles for gripping and lifting by well personnel, often referred to as “roughnecks.” In other cases the slips may be moved into position using a powered mechanical or hydraulic system. Once the pipe is securely held by the slips, additional sections of pipe can be added to/or removed from the drill string.
In some instances, slips comprise two arcuate slip segments hinged on either side of a center arcuate slip segment to form an orifice through which the drill string extends. Each slip segment has an inner surface comprising a plurality of axially milled grooves for receiving a series of vertically stacked gripping elements or inserts. The inserts have roughened surfaces which extend towards and grip the drill string when the slip is engaged with the pipe.
In most slips, the axial grooves are of dovetail cross-section and are machined from the top down to a lower toe area of the slip by a dovetail cutter. The dovetail cutter is circular in shape and as the cutter is milled down to the bottom of the casting, the cutter leaves a radius at the bottom of the dovetail groove. Such a radius experiences high stress concentrations as the axial or “hook” loads of the pipe are transferred through the inserts to the terminal ends of the dovetail grooves. These high stress concentrations often result in deformation or failure of the bottom toe area of the slip segments.
One solution to the high stress caused by the radius at the bottom of the dovetail groove, is to provide a circumferential relief groove for the cutter to pass through at the bottom shoulder of the slip segments such that the radius is eliminated. Half-moon inserts or load supporting rings are then inserted into the relief grooves to provide the dovetail groove a squared terminal end and a flat support surface for the inserts installed along the bottom shoulder. However, because of the large axial loads transferred through the inserts or load rings to the bottom shoulder, many of these inserts or load rings are either pushed out or must be hardened and welded in place to become a more permanent part of the bottom casting.
Although these permanent load supporting devices may improve the performance of the slip, damage to the load supporting devices may require replacement of the entire slip segment. Damage to these load supporting devices may occur due to a variety of reasons. For example, if a slip is used to hold a drilling string large enough to create axial loads close to the slip's rated limit, any additional force caused by the movement of the rig will cause the inserts to jam and overload the load ring. In such instances, the load ring needs to be replaced. If the load ring is permanently welded to the bottom casing, then the entire slip would need to be replaced. Accordingly, it is important that the load rings be removable because they wear and can be overloaded.
In response to the foregoing problems, removable load rings have been developed, such as those manufactured and sold by Varco International, Inc., Orange, Calif. 92868. Specifically, these load rings have been used with slip segments (Part No. 70102-1) for Varco's 1,000 ton elevator spider (Part No. 70100). These load rings are generally semi-circular and installed in relief grooves centrally disposed along the axial dovetail grooves and along the slip's bottom shoulder. These load rings are typically fastened in place by bolts.
Other removable load rings include the type described in U.S. Pat. No. 6,264,395 (the '395 Patent). In an attempt to improve then existing slip assemblies, the '395 patent discloses a slip assembly having slip segments with circumferential grooves cut at reverse angles. The circumferential grooves are adapted to receive complementary shaped surfaces of a load ring to prevent upward slippage of the load ring during loading. The load ring is secured within the grooves by bolts disposed at spaced intervals along the load ring.
While existing removable load rings have been helpful in addressing the problems associated with permanently coupled inserts, the fasteners used to secure these load rings, such as threaded bolts or cotter pins, may provide additional problems. For example, the aforementioned fasteners may become loosened or fail under extreme axial loads and fall into the well bore.
Accordingly, there is a need for a load ring that is removable and easy to install. It is desirable that such a load ring not be secured by fasteners or other means that might loosen and potentially fall into the well bore.